Sweat season is here

Tyre marks on melting asphalt on a sun-battered road at the Dorina crossing on Thursday. (Sanat Kr Sinha)

Calcutta soaked in sweat on Thursday after five days of desert-like hot and dry weather as the minimum relative humidity trebled overnight to 49 per cent from the previous day’s 17 per cent.

The dry wind from central India has subsided and a reverse flow of moist air from the Bay of Bengal was observed on Thursday owing to a cyclonic circulation over neighbouring Jharkhand.

“Moisture-laden air from the Bay is passing through Bengal en route to Jharkhand. This is pushing up the city’s minimum relative humidity, which indicates the moisture content in the air during the day, and making the weather extremely sultry,” said an official at the Met office in Alipore.

The influx of moisture has cleared the decks for thunderstorms but Calcuttans will have to sweat it out until rain drenches the city. “The sky will be partly cloudy over the next two days. Light rain is expected on Friday and a thunderstorm or two on Saturday,” the official added.

The Celsius fell almost two notches from Wednesday’s 37.8 degrees to settle at 36 on Thursday. The maximum temperature was just a degree above normal but the muggy condition raised the discomfort index, which indicates the effect of heat and humidity on the human body, to 63.2 degrees Celsius at 2.30pm, almost nine degrees above the comfort level.

A senior official of the India Meteorological Department in Calcutta said high relative humidity means the surrounding air is saturated with water vapour. “But for thunderstorms, both surface temperature and moisture in the air should be high. The sun has been belting down all right, and heating up the surface, but no weather system had developed over the past few days to generate moisture flow. The course has changed now with the influx of moisture,” he added.

Experts said the conditions were also ripe for Nor’westers from the Chhotanagpur Plateau because of the moisture incursion.

“Nor’westers are formed when the air over the plateau heats up and gains altitude, drawing moisture from Gangetic Bengal in the process. The cycle continues because of the excessive heat and the moisture-laden air becomes hot and buoyant. But at higher altitudes the water vapour condenses and forms tall rain-bearing clouds that bear down as Nor’wester squalls,” a weather expert said.

He said the plateau was getting adequately heated over the past few days and air from Bengal has been rushing in to fill the void. “But the air was bone dry. Now that moisture is moving in, thunderstorms are likely to build in the next two days.”